1. Field of the Invention
The present invention relates to a method for distributing a high frequency clock signal in a computer system and a circuit for providing high voltage output levels from TTL circuitry to drive CMOS circuitry.
2. Description of the Related Art
Personal computers are getting ever faster and as a result are using ever high clock speeds. The speeds have progressed from early personal computers which used clock rates of 1 to 2 MHz to current systems which have an effective clock rate of 33 MHz and use a clocking input signal having a frequency of 66 MHz. While this higher clocking rate allows the microprocessor used in the personal computer to operate at a higher speed, it also creates significant design problems when concerned with the electromagnetic interference (EMI) levels necessary to meet certain standards as developed by the Federal Communications Commissions (FCC). The EMI problem becomes particularly sensitive when these high frequency clock signals must be transmitted over long distances on a circuit board and becomes extremely burdensome and troublesome when the clock signal must pass through a connector.
Connectors are being utilized more frequently in the design of personal computer systems with the ever changing microprocessors available because it has become desirable to place the microprocessor circuitry on a separate plug-in board which is connected through a socket and connector arrangement. The use of a separate board allows interchangeability of basic microprocessor types or addition of more processors in a multiprocessor environment. It is necessary to provide a basic clocking signal to this microprocessor and thus the clock signal must pass through the connector. The inclusion of the connector results in impedance matching and other similar problems. This results in a tendency for reflections to be developed in the clocking signal, which, if bad enough, may cause erroneous clocking signals to be received. The reflections also increase the EMI problems by introducing higher frequency components. While these problems can be solved using relatively conventional techniques, such as shielding and fencing, at frequencies below 50 MHz, when the signals approach 66 MHz conventional techniques suffer undue complexity and increase dramatically in cost. Additionally, the reflections may have an effect on the actual clock skew as developed between various components. The runs must be carefully length and impedance matched if only a single source is to provide clocking and if no local clock deskewing circuitry is provided. In previous designs at these high clock rates matched coaxial lengths of cable was utilized or particular clock deskewing circuitry which included a series of delay lines was used. However, each one of these techniques required that the system be tuned for each particular application, could not use conventional socket techniques and, therefore, were not readily acceptable for mass volume manufacturing.
In yet a further complication the microprocessors conventionally utilized in current personal computers are of CMOS design and thus require relatively high voltage levels at certain inputs to provide a guaranteed high level signal. This is particularly true in clocking circuits, where for example on the Intel Corporation (Intel) 80386 microprocessor the CLK2 input high voltage is specified to be at least 3.7 volts, assuming a nominal supply voltage of 5 volts. However, conventional TTL output high levels are typically 3.4 volts and worst case voltages are more often in the range of 2.5 to 2.7 volts. This clearly results in a problem. CMOS circuitry can be used, but at the higher frequencies, such as 66 MHz, the CMOS circuitry has too much device to device skew to allow a single source to drive multiple clocking signals at the necessary speeds. Therefore, if any sort of deskewing circuitry is utilized at the desired speeds, the circuitry must be developed using bipolar designs because of the instability of conventional CMOS designs. This use of bipolar circuitry results in the aforementioned voltage level problem. Pull up resistors can be utilized with bipolar circuitry but at the desired frequencies are not always effective. Thus, there is a problem properly and inexpensively driving the high level input signals.